This invention relates to an apparatus for continuously hot dip plating one side of a metal strip.
There is a demand for steel sheets which are zinc plated on only one side (so-called one side plated steel sheet), and many processes have been proposed for their manufacture. In this case, careful attention should be paid to insure that molten plating material does not contact the upper non-plating surface of the strip which should not be plated. Otherwise, the resulting strips would not be desired products, or would reduce their saleability.
One of the proposed processes is to upheave or swell the surface of the molten plating material by means of a pump, to contact a horizontally disposed surface of a travelling strip, facing the molten plating material. Such a method is described in Japanese Laid Open Patent Specification No. 53-75,124 (laid open to public inspection in 1978), in which zinc is fed from a pump and spouted from a nozzle installed within a zinc bath, and the zinc surface is upheaved to contact the lower surface of the travelling strip which is horizontally held by a plurality of rolls. In this practice, an inert gas is blown by a nozzle which is disposed above the strip, onto the upper surface of the strip in order to prevent the plating material zinc from contacting the upper non-plating surface of the strip. However, this method requires blowing of a large amount of inert gas under high pressure, as a countermeasure to large changes of widths of the strip, its operation is expensive. Also, due to the counterflow of the inert gas, zinc will inevitably be splashed on the upper surface of the strip
In view of these deficiencies, the present inventor proposed in Japanese Patent Application No. 67,445/79, a process and an apparatus for uniformly plating molten metal on one side of a steel strip, wherein the strip travels horizontally over a surface of a plating bath while the plating molten metal is jetted onto the lower surface of the strip, facing the bath. The molten metal is jetted from a position adjacent the side edges of the strip, in a plating flow running widthwise of the strip and from a position centrally of the strip, in a plating flow running lengthwise of the strip. However, disadvantageously, in this operation, the nozzles must follow the changes in width of the strip as it moves. Accordingly, it was originally planned to move the center and edge nozzles in response to changes in widths. However, as a practical matter, it was found that such movable portions were complicated mechanically, and difficult to implement operationally, and many other problems were caused.
The present invention aims to overcome the foregoing and other deficiencies of the prior art, and encompasses a novel apparatus which produces uniform plating on one side of a metal strip, without any of the plating material being allowed to contact the non-plating surface of the strip regardless of changes in the width of the moving strip. The invention utilizes appropriately shaped nozzles which are positioned at oblique angles to the moving direction of the strip, and which have nozzle outlets which are suitably tilted toward the edges. The oblique angle and angle of tilt can both be suitably adjusted to prevent plating material from creeping around the edges and contact the non-plating upper surface of the strip, and to prevent the plating material from splashing onto the upper surface and cause spots of such plating material on the upper surface. The shape of the nozzles may be adjusted to enable uniform plating on the lower surface and prevent areas of non-plating or irregular plating to occur.